The case of a Minnesota woman accused of setting fires on a college campus last month in a self-professed act of jihad raises questions about whether law enforcement could have done more to stop her.More >>

The case of a Minnesota woman accused of setting fires on a college campus last month in a self-professed act of jihad raises questions about whether law enforcement could have done more to stop her.More >>

Students with their whole lives ahead of them and the teachers who tried to protect them were among the 17 people killed when a gunman opened fire with an AR-15 at a large high school in south Florida.More >>

Students with their whole lives ahead of them and the teachers who tried to protect them were among the 17 people killed when a gunman opened fire with an AR-15 at a large high school in south Florida.More >>

The mass shooting at a Florida high school that left 17 people dead has sparked calls for walkouts, sit-ins and other actions on school campuses nationwide aimed at pushing lawmakers to pass tougher gun laws.More >>

The mass shooting at a Florida high school that left 17 people dead has sparked calls for walkouts, sit-ins and other actions on school campuses nationwide aimed at pushing lawmakers to pass tougher gun laws.More >>

Florida Gov. Rick Scott is vowing to do what he can to keep mentally ill people from getting guns. Attorney General Jeff Sessions also is focusing on intervening with mentally ill people before school shootings happen.

Florida Gov. Rick Scott is vowing to do what he can to keep mentally ill people from getting guns. Attorney General Jeff Sessions also is focusing on intervening with mentally ill people before school shootings happen.

WASHINGTON (AP) - A new study finds that a major trigger of man-made earthquakes rattling Oklahoma is how deep - not just how much - fracking wastewater is injected into the ground.

Scientists analyzed more than 10,000 wastewater injection wells where 96 billion gallons of fluid - leftover from hydraulic fracturing - are pumped yearly. The amount of wastewater injected and the depth are key to understanding the quake outbreak since 2009, they reported in Thursday's journal Science . The quakes included a damaging magnitude 5.8 in 2016, the strongest in state history.

State regulators could cut about in half the number of man-made quakes by restricting deep injections in the ground, said lead author Thea Hincks, an earth scientist at the University of Bristol in the United Kingdom. Companies drilling for oil and gas should not inject waste within 600 to 1500 feet (200 to 500 meters) of the geologic basement. That's the stable harder rock deep underground usually made of metamorphic and igneous rocks.

That region is usually crisscrossed with earthquake faults. The closer you get to the faults, the more likely you are to trigger them, said Stanford University geophysicist Matthew Weingarten, who wasn't part of the study.

Lab experiments show basement rocks are more susceptible to earthquakes because "having fluids in a rock is going to weaken the rock," said co-author Thomas Gernon, a geologist at the University of Southampton.

Previous studies had pinpointed the amount of fluids injected into wells as an issue. Gernon said volume does trigger earthquakes, but when volume levels were reduced the number of quakes didn't drop as much as had been expected. That's because where the stuff is put matters slightly more, he said.

The findings only apply to quakes in Oklahoma, researchers said.

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Follow Seth Borenstein on Twitter at @borenbears . His work can be found here .

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